1. Field of the Invention
The invention relates to the field of signal processing, and more particularly to processing using optical waves. The use of optical waves for transmitting information through optical wave guides, in particular through optical fibers, has multiple advantages particularly because of the very low sensitivity to disturbances of this type of guide. Beyond the simple transmission aspects, a number of applications of this optical signal transmission have revealed interesting properties which will lead to the use of these optical guides also for processing the information transmitted over these guides.
In particular, the use of monomode optical fibers as delay lines is advantageous in particular for processing wide band hyperfrequency signals. In fact, the dispersion performances of optical fibers, more than 100 GHz per km and attenuation, less than 0.5 dB per km, make it possible to envisage passband--delay products greater than 5.times.10.sup.5 with delays of the order of several tens of microseconds.
2. Description of the Prior Art
The performances of monomode fibers for guiding are fully exploitable because of the fact that light sources and adapted III/V semiconductor detectors have been perfected, as well as integrated optical circuits producing the useful functions, for example coupling or modulation.
The applicant company has moreover already described in the information bulletin of its research laboratory, in December 1986, the construction of a 5 GHz band delay line with 5 microseconds delay.
Because of these great possibilities, the use of monomode optical fibers as delay lines for processing has already been envisaged more particularly in an article entitled "Optical fiber delay line signal processing" in the IEEE Transaction on Microwave Theory and Techniques, vol. MTT 33, n.degree. 3, March 1985. This article describes more particularly tapped lines and recirculation lines in which the optical fiber or loops thereof are used for introducing delays in the transmitted signal. The delay lines thus formed combine incoherently the signals from the different tappings provided for this purpose along the delay line.
In theory, the optical carrier could be used for coherent processing, but in this case the relative phases of the optical carrier at each of the signal tappings should be stable up to a fraction of the optical wave length. That would require a narrow and very stable source spectrum; in addition, the optical path differences should also be constant up to a fraction of the wave length. For this reason coherent detection, which depends on the phase of the optical carrier, is not used in devices employing monomode fiber delay lines.
Only the incoherent summation by quadratic detectors is possible and negative weighting, if required, of the signals from the different tappings on a monomode optical fiber delay line, for providing filtering functions, cannot be accomplished optically.